6660IEEETRANSACTIONSONPOWERELECTRONICS,VOL.29,NO.12,DECEMBER2014

DesignandImplementationofaTransformerlessSingle-StageSingle-SwitchDouble-BuckConverterWithLowDC-linkVoltage,HighStep-Down,andConstantInputPowerFactorFeatures

YuChen,Member,IEEE,ZhihaoZhong,andYongKang

Abstract—Thispaperpresentsanewtransformerlesssingle-stagesingle-switch(S4)converterwhichintegratesabuck-typepowerfactorcorrectioncellwithabuck-typedc–dcoutputcellinaspecialway.TheproposedconverterpreservestheadvantagesofexistingtransformerlessS4converters,suchasthelowvoltagestressacrossthedc-linkcapacitor,lowcurrentstressoftheswitch,andhighstep-downinput-to-outputvoltage.Moreover,thepro-posedconverterprovidesanewattractivefeature,i.e.,itsinputpowerfactoralwayskeepsconstantevenwhentheline-andload-conditionsvary.Thesecharacteristicsmaketheproposedconvertercost-ef cient,easy-to-design,andsuitableforthelow-powerandnonisolatedapplications.Theprincipleandanalysisofthepro-posedconverterarepresentedinthispaper,andadesignexampleisalsogiventoshowthevalidityofanalysis.

IndexTerms—LEDdriver,powerelectronics,powerfactorcorrection(PFC),single-stagesingle-switch(S4)converter.

I.INTRODUCTION

M

ANYindustrialapplicationsrequirenotonlyatightdcvoltagetofeedtheirloads,butalsoahigh-inputpower

factor(PF)toreduceharmonicdistortioninthecurrentdrawnfromthegrid.Theserequirementscanbesimplysatis edbyasinglepowerfactorcorrection(PFC)converter[1]–[8].How-ever,itsoutputvoltagehasthehighripplesattwiceofthelinefrequency.Theseripplesgetevenhigherwhentheoutputvoltagebecomeslowerandtheloadbecomesheavier.TheothersolutionisusingaPFCconverterincascadewithadc–dcconverter,withwhichtheinstantaneousinputpowerandthestableoutputpowercanbedecoupledbyanintermediatedc-linkcapacitor[9]–[13].However,suchatwo-stagecon gurationsuffersfromthein-creasedcomponentcount,highcost,andcomplicatedcontrol.

ManuscriptreceivedAugust17,2013;revisedNovember3,2013andJanuary8,2014;acceptedFebruary5,2014.DateofpublicationFebruary14,2014;dateofcurrentversionAugust13,2014.ThisworkwassupportedbytheLite-OnPowerElectronicsTechnologyResearchFundunderGrant2012-04andSpe-cializedResearchFundfortheDoctoralProgramofHigherEducationunderGrant20120142120026.ThispaperwaspresentedinpartattheIEEEEnergyConversionCongressandExposition,2013.RecommendedforpublicationbyAssociateEditorT.Qian.

TheauthorsarewiththeStateKeyLaboratoryofAdvancedElectromagneticEngineeringandTechnologyandtheSchoolofElectricalandElectronicEn-gineering,HuazhongUniversityofScienceandTechnology,Wuhan430074,China(e-mail:ayu03@http://;cobeam@http://;ykang@http://).Colorversionsofoneormoreofthe guresinthispaperareavailableonlineathttp://.

DigitalObjectIdenti er10.1109/TPEL.2014.2306430

Tocompromisethesize,cost,andperformance,manysingle-stagesingle-switch(S4)convertershavebeenproposed.Fromthetopologypointofview,S4converterscanbetreatedasanintegrationversionofthetwo-stagestructure,inwhichtheswitchesinthePFCanddc–dcconverterarecombinedasone.Severaladvantagescanbeobtainedusingthisintegrationapproach:1)thePFCanddc–dccellsshareoneswitch,thesizeandcostcanbereduced;2)thehighinputPFcanbeobtainedautomaticallybythePFCcellwiththediscontinuousconductionmode(DCM),onlytheoutputvoltageneedstoberegulated,thecontrollercanbesimple;and3)theinputandoutputpowercanstillbedecoupledsincetheintermediatedc-linkcapacitorisremained,thereforethevoltageripplesintheoutputarealleviated.Duetothesefeatures,S4convertersareespeciallysuitableforthelow-powerapplicationswithcriticalsizeandcostlimitations.

S4converterswithgalvanicisolationhavebeenwellstud-iedinthepastdecades[14]–[29].However,insomelightingapplications,galvanicisolationisnotmandatorybythesafetyrequirementsorstandards.Forexample,thedriverforemergingsolid-statelightingcouldbenonisolated,astheexistingincan-descentlampsarealsonotisolatedfromthegrid[2],[30]–[32].Insuchacase,transformerlessS4convertersaremorecom-pactandcost-ef cientsolutions.Unfortunately,ifalowoutputvoltageisneeded,anextremelynarrowdutycycleisrequiredinsuchkindofconverterswithoututilizingatransformer,es-peciallyinthehigh-linecondition.Thenarrowdutycycleishardtoberealizedbyalow-costPWMcontrollerandleadstoapooref ciency[3],[33],[34].Therefore,thetopologydesignforsuchtransformerlessS4convertersshouldcompromisenotonlyagoodinputPFandastableoutputvoltage,butalsoahigh-voltagestep-downfeature(i.e.,alowoutputvoltagebutarelativelylargedutycycle).

Recently,severaltransformerlessS4convertershavebeenpresentedtomeettheaforementionedrequirements.Theboost/buck-boostS4converterispresentedin[35].TheboostPFCcellprovidesagoodinputPF,butitalsorequiresahighdc-linkvoltage,whichdegradestheconverter’sstep-downfeature.TheS4converterswithbuck-boostPFCcells[33],[34],[36]reducethedc-linkvoltage,andthevoltagestep-downcanberealizedbyabuck[34],quadraticbuck[33],orbuck-boost[36]cell.However,thesingleswitchintheaforementionedconvert-ersneedstohandlethecurrentsofbothcells,whichleadstothethermalproblemoftheswitch.In[37]and[38],abuck/

0885-8993©2014IEEE.Personaluseispermitted,butrepublication/redistributionrequiresIEEEpermission.

Seehttp:///publicationsstandards/publications/rights/index.htmlformoreinformation.

CHENetal.:DESIGNANDIMPLEMENTATIONOFATRANSFORMERLESSSINGLE-STAGESINGLE-SWITCHDOUBLE-BUCKCONVERTER

6661

Fig.1.

ProposedtransformerlessS4converter.

buck-boostS4converterisproposedtofurtherreducethedc-linkvoltageandenhancethestep-downfeature,andthesingle-switchdoesnotneedtohandlethecurrentsofbothcells.How-ever,sinceitsPFCcellisbucktype,thecircuitmustbecarefullydesignedsothatasatisfactoryinputPFcanbeobtainedinthecaseoftheline-andload-conditionsvariation.

Toovercomethedrawbacksoftheaforementionedtrans-formerlessS4converterbutpreservetheiradvantages,thispaperproposesanewtopologythatintegratesabuckPFCandbuckdc–dccells,asshowninFig.1.Abuckdc–dccellisembeddedinthecircuit(thecomponentshighlightedinblue),wherethedc-linkcapacitorCbistreatedasitsinput;ontheotherhand,ifSisturnedONandDakeepsconducting(theconductionofDawillbeguaranteedbytheparameterdesign,asdiscussedlater),nodeaandnodebwouldhavethesamevoltagepotential,thecomponentsinredconstructanequivalentbuckPFCcell,inwhichvinistreatedasinput,CbinserieswithCoistreatedasoutput.Withsuchintegration,thefollowingbene tscanbeobtained:

1)theequivalentbuckPFCleadstoalowvoltagestressacrossthedc-linkcapacitorCb,theelectrolyticcapacitorswithlowvoltageratingbutlargecapacitycanbeused;2)thesingleswitchSneedstohandleonlythecurrentofthedc–dccell,thecurrentstressislow;

3)integrationoftwobuckcellsleadstothehighstep-downinput-to-outputvoltage;

4)fortheequivalentbuckPFCcell,itsequivalentsink,Cb

andCo,inserieswiththereversedpolarities.Thisstruc-turebringsinanattractiveconstantPFfeature,whichisindependentoftheline-andload-conditions.Thisuniquefeaturefacilitatesthecircuitdesignalot.

Toclearlyexplaintheaforementionedcharacteristics,theop-erationalmodesoftheproposedS4converterarepresentedinSectionII.Then,thecircuitcharacteristicsareanalyzedinSec-tionIII.Accordingtotheanalysis,aneasy-to-followdesignprocedure,adesignexample,anditsexperimentalresultsaregiveninSectionIV.Theconclusionsare nallygiveninSectionV.

II.OPERATIONALMODESOFTHEPROPOSEDS4CONVERTERA.De nitionsofRegionIandRegionII

Tosimplifytheanalysis,allcomponentsaretreatedasideal;thedclinkandoutputcapacitorsarelargeenough,thereforetheirvoltageVbandVoaretreatedasconstantwithinhalf

of

Fig.2.

TwoworkingregionsoftheproposedS4

converter.

Fig.3.KeywaveformsoftheproposedS4converter:(a)inregionIand(b)inregionII.

line-cycleperiod;andaftertherecti er,theinputvoltagevinhasbeenrecti edasvg(θ)=|vin|=|Vpksinθ|,whereVpkisthepeakvoltageofvin,andθisavariablevaluefrom0to2π.Especially,consideringhalfofline-cyclefrom0toπ,vg(θ)canbeexpressedas

vg(θ)=|vin|=Vpksinθ

θ∈[0,π].

(1a)

TheproposedS4converterincludesabuck-typePFCcell,therefore,twoworkingregionscanbefoundwithinhalfofline-cycleperiod,asshowninFig.2.Withtheproposedintegration,theequivalentvoltageVeq=Vb VoactsasthesinkofthePFCcell,therecti erdoesnotconductwhenvg(θ)<Veq,thisintervalisde nedasRegionI.Oppositely,ifvg(θ)>Veq,therecti erconductsandainputcurrentiinappears.Thisintervalisde nedasRegionII.ThestartpointαandendpointβofRegionIIhavebeenmarkedinFig.2,theycanbeexpressedas

α=arcsin

Veq

Vandβ=π arcsinVeq.pkV(1b)

pk

6662IEEETRANSACTIONSONPOWERELECTRONICS,VOL.29,NO.12,DECEMBER2014

Basedonthesede nitions,thekeywaveformsineachregionaregiveninFig.3,andtheoperationalmodesareshowninFig.4,theyareanalyzedasfollows.B.OperationalModesinRegionI

M1(t0 t1):SwitchSisturnedON,thevoltageappliedonL2isVeq=Vb Vo,iL2increaseslinearlyfromzerowiththeslopeVeq/L2;ontheotherhand,thevoltageappliedontherecti erdiodesisvg(θ) Veq.Noticingvg(θ)<Veqinthisre-gion,therecti erisblocked,thereisnocurrent owingthroughL1,iL1=iin=0.ThismodewillpersistuntilSisturnedOFFatt1.Denotingtheduty-cycleasd,thepeakvalueofiL2att1canbecalculatedas

IVL2 pk=eq

LdT2s.(2a)

SinceiL2also owsthroughSandCb,thepeakvaluesofiCbandiSarealsoIL2 pk.

M2(t1 t2):SisturnedOFF,iL2isfreewheelingthroughD2,iL2decreaseslinearlywiththeslopeVo/L2.Thecurrent owingthroughCbandSisnowzero;inthePFCcell,iL1andiinarealsozero,whichisthesameasM1.ThismodewillpersistuntiliL2decreasestozeroatt2.DenotingthedutycycleinthismodeasdDC,itcanbesolvedas

dDC=

Veq

Vd.(2b)

o

M3(t2 t3):OnceiL2decreasestozero,itkeepszerountil

SisturnedONagain.Inthismode,onlytheenergyinCoisreleasedtoRo,thecurrents owingthroughL1,L2,Cb,andSareallzero.

AccordingtotheoperationalmodesinRegionI,theequiv-alentcircuitofthedc–dccellisalsoshowninM1–M3(seeFig.4).Obviously,thedc–dccellisactuallyabuckconverterworkingintheDCM;whilethePFCcellisdeactivatedduringthisregion.

C.OperationalModesinRegionII

M4(t0 t1):WhenSisturnedONinthiscase,thevoltageacrossL2isVeq=Vb Vo,whichisthesameasthatinM1.AndsinceDaalsoconducts,nodesaandbhavethesamevoltagelevels,itmeansthevoltageappliedonL1isvg(θ) Veq.Noticingvg(θ)>Veq,therecti erisconductedandiL1beginstoincreaselinearlyfromzerowiththeslope[vg(θ) Veq]/L1.Itshouldbeemphasizedthat,thismodeispersistingonlywhenDaisconducting.ItmeansiS>iin,orequivalentlyiL2>iL1mustbesatis ed,sothatthedifferencecurrentiDacanbeextractedfromCbthroughDa,andthus,keepsDaconducting.Thisrestrictionissatis edbytheappropriateparameterdesigninSectionIII.Thismodewillpersistuntilt1whenSisturnedOFF.Att1,thepeakcurrentIL2 pkandIS pkarethesameas(2a),andthepeakvalueofiL1andiincanbecalculatedas

IL1 pk=I=v(θ) Vin pk

geqLdTs.

(2c)

1

M5(t1 t2):SisturnedOFF.Inthedc–dccell,iL2isfree-wheelingthroughD2,thevoltageappliedonL2is Vo,there-

foreiL2decreaseslinearlywiththeslope Vo/L2,and nallyreacheszeroatt2.ThecorrespondingdutycycledDCisthesameasthatin(2b).InthePFCcell,iL1isfreewheelingthroughD1,thevoltageappliedonLV1is Veq,iL1decreaseswiththeslope eq/L1.Inthismode,iinkeepszeroandiCbalwaysequalsto iL1.

M6(t2 t3):iL2hasdecreasedtozero,iL1keepsdecreasingbecausethevoltageappliedonL1isstillVeq.iL1will nallyreachzeroatt3.ThecorrespondingdutycycledPFCcanbesolvedas

dv VPFC=

g(θ)eq

Vd.

(2d)

eq

M3(t3 t4):BothiL1andiL2arezero,thecircuitstategoestoM3,asthatinregionI.

TheequivalentcircuitsofthePFCanddc–dccellsinRegionIIarealsodrawninM3–M6(seeFig.4).Itisseenthatthedc–dccellstillworksasabuckconverter,andtheequivalentPFCcellalsobehavesasabuckPFC,whereL1isthebuck-inductor,Veq=Vb Voistheequivalentoutput,andD1isthefreewheel-ingdiode.Itshouldbeemphasizedthat,ifiL1decreasestozero rst,theorderofM5andM6shouldbeexchanged;how-ever,theexpressionsofdDCanddPFCarenotchangedbecausethetwofreewheelingcurrentsarecompletelyindependent.Thispossibilitywillnotberepeatedlydiscussedhere.

III.CIRCUITANALYSIS

Withtheproposedintegration,Veq=Vb VoistreatedastheequivalentsinkofthePFCcell.Tofacilitatetheanalysis,thepeak-input-to-equivalent-sinkratioMpeandtheinductionratioLare rstde nedasfollows:

MVpe=

eqVandL=L2

.(3)

pkL1

Basedonthede nitions,thecircuitcharacteristicsthatareindependentanddependentontheline-andload-conditionsareanalyzed,respectively.

A.Line-andLoad-IndependentCharacteristics

1)Single-ValuedRelationBetweenLandMpe:IneitherRegionIorII,iCb,thecurrents owingthroughthedc-linkcapacitorCbwithinoneswitchingperiodaretriangle-form,theiraveragevaluescanbecalculatedas

iCb I 1

Ts=2dIL2 pkand iCb II Ts

=12d(I1

L2 pk IL1 pk) 2

dPFCIL1 pk.(4a)Inthesteadystate,theaveragecurrent owingthroughCb

withinhalfofline-cycleperiodshouldbezero

α

β

2

b ICb II0 iC Tsdθ+

α

i Tsdθ=0.

(4b)

CHENetal.:DESIGNANDIMPLEMENTATIONOFATRANSFORMERLESSSINGLE-STAGESINGLE-SWITCHDOUBLE-BUCKCONVERTER

6663

Fig.4.OperationalmodesoftheproposedS4converter.

BasedontheanalysisinSectionII,(4b)canbe nallycalcu-latedas

L=

2

2πMpe

followingrestriction:

IL2 pk>IL1 pk.

as

2πMpe(1 Mpe)

<1.

2π 2arcsinMpe 2·Mpe1 Mpe

(5b)(5a)

π 2arcsinMpe 2·Mpe

21 Mpe

.(4c)

Using(2a),(2c),and(4c),(5a)canbeequivalentlyexpressed

SinceMpe>0,Lisasingle-valuedfunctionthatonlyrelatesto

Mpe.NoticingthatLisactuallyacircuitparameterdeterminedbythedesigner,theconclusion(4c)exposesthatthecircuitstateMpebecomesconstantonceLischosen,regardlessoftheline-andload-conditionsvariation.

2)CircuitRestrictionforDaConduction:ToensurethatDaisconducting,theconditioniL2>iL1mustbesatis edduringM4.SinceiL1andiL2increasefromzerototheirpeakvaluesIL1 pkandIL2 pk,respectively,iL2>iL1isequivalenttothe

De ningtheexpressionontheleftsidein(5b)asFDa,theMpeversusFDacurvecanbecalculatedasshowninFig.5(a).ItisseenthatifMpe<0.416,FDawillbesmallerthan1andthus,thecondition(5b)willalwaysbesatis ed.Ontheotherhand,therelationbetweenLandMpein(4c)canbedrawnasshowninFig.5(b).SubstitutingMpe=0.416into(4c),theequivalentrestrictionL<0.712canalsobeobtained.

6664IEEETRANSACTIONSONPOWERELECTRONICS,VOL.29,NO.12,DECEMBER

2014

Fig.5.Line-andload-independentcharacteristics(a)MpeversusFDa,(b)MpeversusL,(c)Mpeversusγ,and(d)MpeversusPF.

3)LengthofRegionIIandtheInputPF:γ,thelengthofregionII(seeFig.2)canbeexpressedas

γ=π 2α=π 2arcsinMpe.

(6a)

Obviously,γisalsoasingle-valuedfunctionthatonlyrelatestoMpebutindependentoftheline-andload-conditions.TheMpeversusγcurveisshowninFig.5(c).ItisseenthatthesmallerMpeleadstothehigherγ,andthus,themoresinusoidal-likeaverageinputcurrent.Also,itiswellknownthatγisdirectlylinkedtotheinputPFinthebuckPFC;thus,itcanbeexpectedthattheinputPFisalsoasingle-valuedfunctionofMpe.Toprovethis,theaverageinputcurrentwithinoneswitchperiodinregionIIiscalculatedas

iin II Ts=

1

dIL1 pk.2

(6b)

ThecalculationshowsthatthePFisalsoasingle-valuedfunctionofMpe.TheMpeversusPFcurveisdrawninFig.5(d).ItisseenthatthelargerMperesultsinthelowerinputPF.However,evenMpeischosenasitsmaximalvalue0.416(tosatisfytherestrictionforDaconduction),PF=0.961isstillhighenough[38].

B.Line-andLoad-DependentCharacteristics

TheaforementionedpartpresentsthecircuitcharacteristicsthatonlyrelatetoMpe(ortheinductionratioL);however,othervariablessuchasthedc-linkvoltage,duty-cycle,voltage,andcurrentstressesofthedevicesaredependentnotonlyonMpe,butalsoontheline-andload-conditions.Theseissuesarediscussedinthispart.

1)DC-LinkVoltageVb:NoticingVeq=Vb Vo,thedc-linkvoltageVbcanbeexpressedas

Vb=MpeVpk+Vo.

(7)

Inhalfoftheline-cycleperiod,theaverageinputpowerandthermsinputcurrentarecalculatedas

1β

Pin=Vpksin(θ)· iin II Tsdθand

πα

1β

Iinrms=( iin II Ts)2dθ.(6c)

παAccordingtothede nitionoftheinputPF,thefollowingexpressioncanbeobtained:

√γ 2Mpe1 M2pe2Pin

PF== . VpkIinrms

πγ1+2M21 M2pe 6πMpepe

(6d)

ItisseenthatVbisproportionaltotheinputvoltageVpk

withthescaledcoef cientMpeandshiftedbyadcbiasVo.EvenVpkandVohavebeendetermined,asatisfactoryVbcanstillbeobtainedbychoosingaproperMpe.ToillustratethevariationofVb,(7)canbecalculatednumericallyasFig.6(a).InFig.6(a),thex-axisdenotesthevariationofVpkfrom127to373V(correspondingtothermsinputvoltagefrom90to264V);they-axisdenotesthevariationofVofrom5to48V,whichmeetsmostrequirements;andthez-axisdenotesthevalueofVbunderacertainoperationalpoint(Vpk,Vo).ItisseenthatifMpeischosenas0.22or0.31(i.e.,PF=0.99orPF=0.98),Vb<160Vcanberealizedinmostoftheoperationalpoints,anelectrolyticcapacitorwith160Vratingvoltagethuscanbe

CHENetal.:DESIGNANDIMPLEMENTATIONOFATRANSFORMERLESSSINGLE-STAGESINGLE-SWITCHDOUBLE-BUCKCONVERTER

6665

Fig.6.Line-andload-dependentcharacteristics:(a)dc-linkvoltageand(b)DCMrestriction.

used;andifMpeisselectedas0.37or0.41(i.e.,PF=0.97or0.96),Vb>160VmayhappenwhenthecircuitgoesintocertainoperationalregionwherebothVpkandVoarehigh.2)DCMRestriction:Fig.6(a)showsthatalowdc-linkvolt-agecanbeobtainedbyselectingasmallMpe,atthesametimeahigherinputPFcanalsobeobtained;however,theavailableminimalMpeislimitedbytheDCMcondition.Firstofall,toensuretheDCMinbothPFCanddc–dccells,iL1andiL2mustdecreasetozerowithinoneswitchingperiod.Thefollowingrelationsthusshouldbesatis ed:

d+dDC<1andd+dPFC<1.

(8a)

Substituting(2b)and(2d)into(8a),therestrictionscanbeexpressedas

d<

Vo

VVandd<M(8b)

o+Mpepk

pe.

And,sinceboththePFCanddc–dccellsneedtobemain-tainedintheDCM,thetworestrictionsmustbesatis edsyn-

chronously.Theminimalvalueofthemmustbeselectedastheboundaryrestriction

dVDCM=min

o

VM,Mo+peVpe

pk

M2peVo= Mpe1 M<peVpk

VoM2.(8c)

peVoVo+MpeVpk1 Mpe≥

VpkOntheotherhand,supposingthatthecircuitisideal,allthe

inputpowerPinwill nallydissipateontheload,i.e.,Pin=Po.UtilizingthecalculationofPinin(6c),theduty-cycledcanbesolvedas

√

d=2PofsL2

M.(8d)

peVpkEquation(8c)showsthattheDCMrestrictiondDCMdependsonMpeandtheline-andload-conditionsVpkandVo,itsvari-ationcanbeillustratedasFig.6(b).ItisseenthatthemaximalboundarydutycycleislimitedatdDCM=MpewheninputVpkislowandoutputVoishigh.Therefore,althoughasmallMpeleadstoalowdc-linkvoltageandhighinputPF,itresultsinasevererDCMrestrictionanddegradesthestep-downfeature.Forexample,ifPF=0.99isdesired,dDCM=Mpe=0.22mustbeselected,asshowninFig.6(b)(seethedashedregion).Asaresult,theactualdutycyclemustbelowerthan0.22evenwheninputVpkislowandoutputVoishigh.Referto(8d),ei-therPo,fs,orL2needstobedecreased,whichlimitstheratingpower,increasesthesizeofinductors,orincreasesthecurrentstressofthecomponents.Therefore,theselectionofMpemustbetradedoffduringthedesignprocess.

3)VoltageandCurrentStressesoftheSemiconductors:Ac-cordingtotheoperationalmodes,thevoltagestressesareeasytoobtain(seeTableI)andthus,wouldnotbeexplainedhere.However,thermscurrentsofthesedevicesarealittlebitcom-plicated.Here,thecurrentstressonthesingleswitchistakenasexample.First,thermscurrentofthesingleswitchwithinoneswitchingperiodcanbeexpressedas

iM

s rms Ts=

peVpkd3

L.(9a)2fs3Thermscurrentwithinhalfline-cycleperiodcanfurtherbe

obtainedas

I1 πM s rms=peVd3π is rms 2θ=pk0TsdL.(9b)2fs3Theresultsof(9a)and(9b)aretotallythesamebecausethe

singleswitchonlyneedstohandlethecurrentofthedc–dccell,andisnotaffectedbytheinputvoltagevariation.Withthesim-ilarmanners,IDa rms,ID1 rms,andID2 rms(thermscurrents owingthroughDaandthetwofreewheelingdiodesD1andD2)canalsobesolved,asgiveninTableI.Itisinterestingto ndthat,alltheseexpressionscanbeexpressedinsuchformsthatincludetheIs rmsterm.Physically,thisisbecausethesin-gleswitchSissharedbybothcells,therefore,current owing

6666IEEETRANSACTIONSONPOWERELECTRONICS,VOL.29,NO.12,DECEMBER2014

TABLEI

VOLTAGEANDCURRENTSTRESSOFTHEMAINS

EMICONDUCTORS

TABLEII

LOOK-UPTABLEFORTHERAPIDD

ESIGN

TABLEIII

SPECIFICATIONOFTHEP

ROTOTYPE

throughShasthenaturalrelationstothose owingthroughothercomponents.C.Summarization

ForatransformerlessS4converter,theparameterssuchasinputPF,inductionratio,dc-linevoltage,andduty-cycleareconcernedfordesign,buttheyareusuallycouplingandcompli-cated;however,theaforementionedanalysisshowsthatthere-lationsoftheseparametersintheproposedS4converterbecomequitesimple,assummarizedinTableII.TaketheintermediatevariableMpeasabridge,therelationsofinputPF,Landγcanbedeterminedimmediately,andthepossiblemaximalboundaryduty-cycledDCMandthedc-linkvoltageVbcanalsobeknown.Inotherwords,oncetheinductionratioLisdetermined,allthePF,thevoltagestep-downfeature,andthedc-linkvoltagestresscanbeestimateddirectly,regardlessofload-andline-variations.Theseuniquefeaturesfacilitatethecircuitdesignalot.How-ever,itshouldbeemphasizedagainthatalthoughthesmallerMpeleadstothehigherPFandlowerdc-linkvoltage,italsoresultsinthenarrowerrangeofdutycyclefrom0todDCM=Mpe,whichdegradesthestep-downfeature,thisfactormustbecompromisedduringdesign.

IV.DESIGNEXAMPLEANDEXPERIMENTALRESULTSA.PrototypeDesign

Accordingtotheanalysis,adesignexampleisgiveninthissection.Thespeci cationsoftheprototypearegiveninTableIII.

ThedesignprocedureissimpleduetotheuniquefeaturesoftheproposedS4converter.

Step1:SincePF=0.98isexpected,theinductionratioL=

0.31canbeobtaineddirectlyfromTableII.

Step2:FromTableII,themaximaldc-linkvoltageVb=

133Vcanbecalculated.Therefore,a160-Velec-trolyticcapacitorwithlargecapacitycanbeselected.

Step3:AccordingtoTableII,thepossiblemaximaldDCM

=0.31canalsobeknown;ontheotherhand,sincefsandPoarealsoknown,theactualdutycycledcanalsobedesignedaccordingto(8d).AsshowninFig.7(a),ifL2=7or14μHischosen,thedcurvewillalwaysbebelowthedDCMcurveevenwhenVpkchangesfrom127to373V(correspondingtothermsvoltagechangesfrom90to264V).ItimpliesthattheDCMconditionissatis edintheentireinputrange.However,ifL2is21or28μH,partsofdcurvewillbehigherthandDCMcurve,whichgoesagainsttheDCMcondition.Therefore,L2=14μHischosenheretosatisfytheDCMconditions.L2=14μHalsoleadstoamaximaldutycycleunderlow-linecondition(approachingto0.3),thecurrentstressesofthedevicescouldbealleviated.

Step4:OnceL2isdetermined,L1canbecalculatedasL1=

L2/L=45μH

Step5:SinceL1andL2aredetermined,thermscurrentsof

themainsemiconductorscanbecalculatedaccordingtoTableI.AsshowninFig.7(b),themaximalvalues

CHENetal.:DESIGNANDIMPLEMENTATIONOFATRANSFORMERLESSSINGLE-STAGESINGLE-SWITCHDOUBLE-BUCKCONVERTER

6667

Fig.7.Parametercalculation:(a)duty-cycleselectionand(b)currentstress

calculation.

Fig.8.Prototype:(a)maincomponentselectionsand(b)photographof

prototype.

Fig.9.Experimentalwaveformswithinseveralswitchingperiod:(a)and(c)fullloadand90-Vrmsvoltageinput;(b)and(d)fullloadand264-Vrmsvoltageinput.

ofIs rms=5.35A,IDa rms=4.27A,andID1 rms=3.29AappearwhenVpkisminimal;whilethevalueofID2 rms=7.7Akeepsconstantintheentireoperationalrange.

6668IEEETRANSACTIONSONPOWERELECTRONICS,VOL.29,NO.12,DECEMBER

2014

Fig.10.Experimentalwaveformswithinseveralline-cycleperiodinfullload:(a)90-Vrmsvoltageinput,(b)110-Vrmsvoltageinput,(c)220-Vrmsvoltageinput,and(d)264-Vrmsvoltageinput.Step6:Finally,themaximalvoltagesofthesecomponents

canalsobesolvedaccordingtoTableI,thecalcula-tionsshowthatSandDaneedtohandleapossiblepeakvoltage506V,D2needstohandlethe

dc-link

Fig.11.Experimentalwaveformswithinoneline-cycleperiodinfullload:(a)90-Vrmsvoltageinput,(b)110-Vrmsvoltageinput,(c)220-Vrmsvoltageinput,and(d)264-Vrmsvoltageinput.

voltage133V,andD1needstohandlethemaximalinputvoltage373V.

Accordingtothecalculation,thecomponentselectionandthephotographoftheprototypearegiveninFig.8.AsimplevoltagefeedbackloopbasedonUC3842isusedandasimplePI

CHENetal.:DESIGNANDIMPLEMENTATIONOFATRANSFORMERLESSSINGLE-STAGESINGLE-SWITCHDOUBLE-BUCKCONVERTER

6669

Fig.12.Measurementresults:(a)dc-linkvoltage,(b)dutycycle,(c)rmscurrent owingthroughS,(d)inputPF,(e)THD,and(f)ef ciency.

controllerisdesignedtoobtainthedesired19Voutputvoltage.Sincethispaperfocusesonthecircuitcharacteristicsanalysis,someadvancedspeci cations,suchastransientresponseandhold-uprequirement,arenotconsideredintheprototype.B.ExperimentalResults

ThewaveformsofiL1,iL2,Vgs,andVds(thegatinganddrain–sourcevoltageofthesingleswitchS)withinseveralswitchingperiodaregiveninFig.9.InFig.9(a),theoutputisfullloadandthelowest127-Vinputvoltage(i.e.,90Vrms)isapplied,thedutycycleismaximal,however,iL1andiL2stillworkintheDCM;inFig.9(b),theoutputisstillfullloadbutthehighest373-Vinputvoltage(i.e.,264Vrms)isapplied,thedutycyclebecomesminimalandthemaximalvoltagestressVds≈510Vappears.Andinthebothcases,therestrictioniL2>iL1isalwayssatis edwhenSisturnedON,whichensurestheconductionofDa.InFig.9(c)and(d),thewaveformsofiSandiL2arefurthergiven.ItisseenthatwhenSisturnedON,thecurrent owingthroughiSandiL2arethesame,itveri estheconclusionthatthesingleswitchSonlyneedstohandlethecurrentofthedc–dccell.

Thewaveformsofiin ave[i.e.,theinputcurrentthathasbeensmoothedbytheLC lter,seeFig.8(a)],VbandVowithinseveralline-cycleperiodsaregiveninFig.10.TheoutputisfullloadandfourtypicalcaseswhereVpk=127,155,311,and373V(i.e.,90,110,220,and264-Vrmsvoltage)aregiven.Itisseenthatinallcases,iin avehasbeenshapedassinusoidal-likeform,andthemaximalVb=135VappearsinVpk=373V;inallthecases,theoutputvoltageVohasbeenregulatedas

19V.Fig.11furthergivesthedetailedwaveformsofVeq,vin,andiin aveunderVpk=127,155,311,and373Vsituations.ItisclearlyseenthatwhenVpkincreases,Veqalsoincreases,buttheirratioMpealmostkeepsconstant,thereforeγ(thelengthofregionII)alsokeepsconstant.Accordingtothewaveforms,γ=4/5π=2.51(rad)ismeasuredinallconditions,whichwellmatchesthecalculationinTableII.

InFig.12(a),(b),and(c),themeasuredandcalculatedVb,d,andIs rmsinfullloadarecompared;theymatcheachotherwell.InFig.12(d),themeasuredinputPFsunderdifferentline-andload-conditionsarealsogiven.ThemaximalandminimalinputPFsare0.975and0.96,respectively.Thedeviationbe-tweenmeasuredresultsandthedesired0.98ismainlycausedbythenonidealfactorswhicharenotconsideredintheanalysis,suchastheresistancesoftheswitchandinductors,thevoltagedropsofthediodes,thevoltage uctuationsonthecapacitors,andtheimperfectsinusoidalinputvoltageobtainedinthelab.Thesefactorsbecomedominantwhentheinputcurrentislow,therefore,thePFslightlydecreaseswheneitherthelinevoltageincreasesortheloaddecreases.However,thedeviationbetweenthemeasurementanddesiredvalueislessthan1%innormalcondition(PF=0.974infullloadand220Vrmsinput),andlessthan2%evenintheworstcase.Suchaccuracyissatisfac-toryenoughtoensuretheTHDoftheinputcurrentcomplianttoIEC61000-3-2,ClassCstandardintheentireoperationalrange,asshowninFig.12(e).

Finally,theef ciencyoftheprototypeisgiveninFig.12(f).Itisseenthatthebetteref ciencyperformanceappearsinmediateloadandthenormal110and220Vconditions.Themaximalef ciencyis89%.SincetheproposedS4converterisactually

6670IEEETRANSACTIONSONPOWERELECTRONICS,VOL.29,NO.12,DECEMBER2014

anintegrationversionofthetwo-stagestructure,itsef ciencyisrelativelylowascomparedtothetypicalsinglebuckPFCsforlightingapplication(forexample,the12-W/22.5-V-tappedinductorbuckPFCin[2]providesanef ciencyof90.2%,the13-W/19.8-Vresonance-assistedbuckPFCin[2]providesanef ciencyof92.8%,andthe150-W/145-VbuckPFCin[9]providesanef ciencyupto96%).However,comparedtoothertransformerlessS4converters,the89%ef ciencyissatisfactory(forexample,thetransformerlessS4convertersin[33]–[38]havethesimilarpowerlevelfrom20to100W,andtheiref -ciencyare77%,80.5%,85%,89%,and89.5%,respectively).ThemainlossesoftheproposedS4convertercomefromthetwiceenergyprocessingbythePFCanddc–dccells,thecon-ductionlossoftheadditionaldiodeDa,andthehighrmscurrentintheDCM.

Theexperimentsalsoshowthat,toensuretheconstantinputPFfeatureandacceptablevoltageripples,bothCbandCoarerelativelylargesothatVoandVeqkeepalmostconstant,thisisoneofthemaindisadvantagesoftheproposedS4converter.Fortunately,thecapacitorswithlowratingvoltagebutlargecapacitance(forexample,1mF/160Vand1mF/35V,here)canbeutilizedduetothelowdclinkandoutputvoltage.Besides,theoutputground(Coground)intheproposedS4converteris oating.The oatinggroundoutputisallowedinthelightingapplicationsbecausetheirloads(forexample,theLEDs)aretotallystandalone[2],[31],[37],[38].However,the oatinggroundcontributesanadditionalcommon-mode(CM)noisesource,andtheCMnoisewillinjectintothegridthroughtherecti erdiodes.Nevertheless,thisproblemcanbesolvedbyinsertingaCMchokebetweenthegridandtherecti er,asthosein[8],[9],and[39].SincetheCMchokeissmallinsuchpowerlevel,itwillnotincreasethesizeandcostdistinctly.

V.CONCLUSION

ThispaperproposesatransformerlessS4converterintegrat-ingabuckPFCcellandabuckdc–dccellinanovelway.Theproposedintegrationleadstosomeuniqueline-andload-independentfeatures,whichfacilitatethecircuitdesign.Ac-cordingtotheanalysis,thefollowingconclusionsareobtained:1)duetotheintegrationoftwobuckcells,ahighstep-downfea-tureisobtained.Inlow-lineandfull-loadconditions,thedutycycleoftheprototypeisupto0.3,andinhigh-lineconditionisstill0.1;2)thebuckPFCcellleadstoalowdc-linkcapacitorvoltagenothigherthan160V,thereforeanelectrolyticcapacitorwithlow-voltageratingbutlargecapacitycanbeused,3)thesin-gleswitchonlyhandlesthecurrentofthedc–dccell,thecurrentstressislow,4)thelengthofregionIIkeepsconstant(γ=4/5π)regardlessofline-andload-conditionsvariation;theinputPFisveryclosedtothedesiredvalue,andtheinputharmoniccurrentsarecompatiblewiththeIEC61000-3-2,ClassCstandardintheentireoperationalrange.Duetothesebene ts,theproposedS4convertercanbetreatedasacost-ef cientcandidateforthelow-power,nonisolated,andstandalone-loadapplicationswhereahighinputPFandastableoutputarerequired.

REFERENCES

[1]X.Xie,J.Wang,C.Zhao,Q.Lu,andS.Liu,“Anoveloutputcurrent

estimationandregulationcircuitforprimarysidecontrolledhighpowerfactorsingle-stage ybackLEDdriver,”IEEETrans.PowerElectron.,vol.27,no.11,pp.4602–4612,Nov.2012.

[2]http://mar,M.Fernandez,M.Arias,M.M.Hernando,andJ.Sebastian,

“Tapped-inductorbuckHB-LEDAC-DCdriveroperatinginboundaryconductionmodeforreplacingincandescentbulblamps,”IEEETrans.PowerElectron.,vol.27,no.10,pp.4329–4337,Oct.2012.

[3]X.Qu,S.-C.Wong,andC.K.Tse,“Resonance-assistedbuckconverter

forof inedrivingforpowerLEDreplacementlamps,”IEEETrans.PowerElectron.,vol.26,no.2,pp.532–540,Feb.2011.

[4]S.Moon,G.-B.Koo,andG.-W.Moon,“Anewcontrolmethodofinter-leavedsingle-stage ybackAC-DCconverterforoutdoorLEDlightingsystem,”IEEETrans.PowerElectron.,vol.28,no.8,pp.4051–4062,Aug.2013.

[5]X.Wu,Z.Wang,andJ.Zhang,“Designconsiderationsfordual-output

quasi-resonant ybackLEDdriverwithcurrent-sharingtransformer,”IEEETrans.PowerElectron.,vol.28,no.10,pp.4820–4830,Oct.2013.

[6]H.-H.Chou,Y.-S.Hwang,andJ.-J.Chen,“Anadaptiveoutputcurrent

estimationcircuitforaprimary-sidecontrolledLEDdriver,”IEEETrans.PowerElectron.,vol.28,no.10,pp.4811–4819,Oct.2013.

[7]C.-Y.Oh,D.-H.Kim,D.-G.Woo,W.-Y.Sung,Y.-S.Kim,andB.-K.Lee,

“Ahigh-ef cientnonisolatedsingle-stageon-boardbatterychargerforelectricvehicles,”IEEETrans.PowerElectron.,vol.28,no.12,pp.5746–5757,Dec.2013.

[8]W.Xinke,J.Yang,J.Zhang,andM.Xu,“Designconsiderationsof

soft-switchedbuckPFCconverterwithconstanton-time(COT)control,”IEEETrans.PowerElectron.,vol.26,no.11,pp.3144–3152,Nov.2011.

[9]X.Wu,J.Yang,J.Zhang,andZ.Qian,“Variableon-time(VOT)-controlled

criticalconductionmodebuckPFCconverterforhigh-inputAC/DCHB-LEDlightingapplications,”IEEETrans.PowerElectron.,vol.27,no.11,pp.4530–4539,Nov.2012.

[10]L.R.Nerone,“AutoswitchingLEDdriver,”IEEETrans.PowerElectron.,

vol.27,no.8,pp.3834–3842,Aug.2012.

[11]M.Arias,M.Fernandez,http://mar,D.Balocco,A.A.Diallo,and

J.Sebastian,“High-ef ciencyasymmetricalhalf-bridgeconverterwithoutelectrolyticcapacitorforlow-output-voltageAC-DCLEDdrivers,”IEEETrans.PowerElectron.,vol.28,no.5,pp.2539–2550,May2013.

[12]F.Zhang,J.Ni,andY.Yu,“HighpowerfactorAC-DCLEDdriverwith

lmcapacitors,”IEEETrans.PowerElectron.,vol.28,no.10,pp.4831–4840,Oct.2013.

[13]F.Musavi,M.Craciun,D.S.Gautam,W.Eberle,andW.G.Dunford,“An

LLCresonantDC-DCconverterforwideoutputvoltagerangebatterychargingapplications,”IEEETrans.PowerElectron.,vol.28,no.12,pp.5437–5445,Dec.2013.

[14]H.-Y.LiandH.-C.Chen,“Dynamicmodelingandcontrollerdesignfora

single-stagesingle-switchparallelboost- yback- ybackconverter,”IEEETrans.PowerElectron.,vol.27,no.2,pp.816–827,Feb.2012.

[15]C.QiaoandK.M.Smedley,“Atopologysurveyofsingle-stagepowerfac-torcorrectorwithaboosttypeinput-current-shaper,”IEEETrans.PowerElectron.,vol.16,no.3,pp.360–368,May2001.

[16]M.T.Madigan,R.W.Erickson,andE.H.Ismail,“Integratedhigh-quality

recti er-regulators,”IEEETrans.Ind.Electron.,vol.46,no.4,pp.749–758,Aug.1999.

[17]D.D.-C.Lu,H.H.-C.Iu,andV.Pjevalica,“Single-stageAC/DCboost-forwardconverterwithhighpowerfactorandregulatedbusandoutputvoltages,”IEEETrans.Ind.Electron.,vol.56,no.6,pp.2128–2132,Jun.2009.

[18]M.H.L.Chow,Y.S.Lee,andC.K.Tse,“Single-stagesingle-switch

isolatedPFCregulatorwithunitypowerfactor,fasttransientresponse,andlow-voltagestress,”IEEETrans.PowerElectron.,vol.15,no.1,pp.156–163,Jan.2000.

[19]H.Wei,I.Batarseh,G.Y.Zhu,andP.Kornetzky,“Asingle-switchAC-DCconverterwithpowerfactorcorrection,”IEEETrans.PowerElectron.,vol.15,no.3,pp.421–430,May2000.

[20]L.Huber,J.Zhang,M.M.Jovanovic,andF.C.Lee,“Generalizedtopolo-giesofsingle-stageinput-current-shapingcircuits,”IEEETrans.PowerElectron.,vol.16,no.4,pp.508–513,Jul.2001.

[21]C.-M.Wang,“Anovelsingle-switchsingle-stageelectronicballastwith

highinputpowerfactor,”IEEETrans.PowerElectron.,vol.22,no.3,pp.797–803,May2007.

CHENetal.:DESIGNANDIMPLEMENTATIONOFATRANSFORMERLESSSINGLE-STAGESINGLE-SWITCHDOUBLE-BUCKCONVERTER6671

[22]M.A.D.Costa,J.M.Alonso,J.C.Miranda,J.Garcia,http://mar,

“Asingle-stagehigh-power-factorelectronicballastbasedonintegratedbuck ybackconvertertosupplymetalhalidelamps,”IEEETrans.Ind.Electron.,vol.55,no.3,pp.1112–1122,Mar.2008.

[23]J.M.Alonso,M.A.DallaCosta,andC.Ordiz,“Integratedbuck- yback

converterasahigh-power-factoroff-linepowersupply,”IEEETrans.Ind.Electron.,vol.55,no.3,pp.1090–1100,Mar.2008.

[24]J.-L.Lin,M.-Z.Chang,andS.-P.Yang,“Synthesisandanalysisforanovel

single-stageisolatedhighpowerfactorcorrectionconverter,”IEEETrans.CircuitsSyst.,vol.52,no.9,pp.1928–1939,Sep.2005.

[25]T.F.WuandY.K.Chen,“Analysisanddesignofanisolatedsingle-stage

converterachievingpower-factorcorrectionandfastregulation,”IEEETrans.Ind.Electron.,vol.46,no.4,pp.759–767,Aug.1999.

[26]Y.-C.LiandC.-L.Chen,“Anovelsingle-stagehigh-power-factorAC-to-DCLEDdrivingcircuitwithleakageinductanceenergyrecycling,”IEEETrans.Ind.Electron.,vol.59,no.2,pp.793–802,Feb.2012.

[27]A.R.Ghanbari,E.Adib,andH.Farzanehfard,“Single-stagesingle-switch

powerfactorcorrectionconverterbasedondiscontinuouscapacitorvoltagemodebuckand ybackconverters,”IETPowerElectron.,vol.6,pp.146–152,2013.

[28]Y.Hu,L.Huber,andM.M.Jovanovic,“Single-stage,universal-input

AC/DCLEDdriverwithcurrent-controlledvariablePFCboostinductor,”IEEETrans.PowerElectron.,vol.27,no.3,pp.1579–1588,Mar.2012.[29]C.K.Tse,M.H.L.Chow,andM.K.H.Cheung,“AfamilyofPFCvolt-ageregulatorcon gurationswithreducedredundantpowerprocessing,”IEEETrans.PowerElectron.,vol.16,no.6,pp.794–802,Nov.2001.[30]M.Ryu,J.Baek,J.Kim,S.Park,andH.Kim,“Electrolyticcapacitor-less,

nonisolatedPFCconverterforhigh-voltageLEDsdriving,”inProc.IEEEInt.Conf.PowerElectron.ECCEAsia,2011,pp.499–506.

[31]X.Xie,C.Zhao,Q.Lu,andS.Liu,“Anovelintegratedbuck- yback

nonisolatedPFCconverterwithhighpowerfactor,”IEEETrans.Ind.Electron.,vol.60,no.12,pp.5603–5612,Dec.2013.

[32]C.-S.Moo,Y.-J.Chen,andW.-C.Yang,“Anef cientdriverfordimmable

LEDlighting,”IEEETrans.PowerElectron.,vol.27,no.11,pp.4613–4618,Nov.2012.

[33]M.A.Al-Saffar,E.H.Ismail,andA.J.Sabzali,“Integratedbuck-boost-quadraticbuckPFCrecti erforuniversalinputapplications,”IEEETrans.PowerElectron.,vol.24,no.12,pp.2886–2896,Dec.2009.

[34]E.H.Ismail,A.J.Sabzali,andM.A.Al-Saffar,“Buck-boost-typeunity

powerfactorrecti erwithextendedvoltageconversionratio,”IEEETrans.Ind.Electron.,vol.55,no.3,pp.1123–1132,Mar.2008.

[35]S.K.KiandD.D.C.Lu,“Implementationofanef cienttransformerless

single-stagesingle-switchAC/DCconverter,”IEEETrans.Ind.Electron.,vol.57,no.12,pp.4095–4105,Dec.2010.

[36]J.M.Alonso,J.Vina,D.G.Vaqero,G.Martinez,andR.Osorio,“Analysis

anddesignoftheintegrateddoublebuck-boostconverterasahigh-power-factordriverforpower-LEDlamps,”IEEETrans.Ind.Electron.,vol.59,no.4,pp.1689–1697,Apr.2012.

[37]D.LuandS.Ki,“Light-loadef ciencyimprovementinbuck-derived

single-statesingle-switchPFCconverters,”IEEETrans.PowerElectron.,vol.28,no.5,pp.2105–2110,May2013.

[38]S.-K.KiandD.D.-C.Lu,“Ahighstep-downtransformerlesssingle-stage

single-switchAC/DCconverter,”IEEETrans.PowerElectron.,vol.28,no.1,pp.36–45,Jan.2013.

[39]H.Choi,“Interleavedboundaryconductionmode(BCM)buckpower

factorcorrection(PFC)converter,”IEEETrans.PowerElectron.,vol.28,no.6,pp.2629–2634,Jun.

2013.

YuChen(S’09–M’11)receivedtheB.E.andPh.D.degreesinelectricalandelectronicengineeringfromtheHuazhongUniversityofScienceandTechnology,Wuhan,China,in2006and2011,respectively.

FromMarch2008toMarch2009,hewasanIn-terninGEGlobalResearchCenter,Shanghai,China.InSeptember2011,hejoinedtheHuazhongUni-versityofScienceandTechnologyasaLecturer.Hisresearchinterestsincludethepowerelectronicconvertertopologies,soft-switchingtechniques,con-vertermodeling,thefaultdiagnosistechniques,and

thewindenergypower-conversion

system.

ZhihaoZhongwasborninGuangdongProvince,China,in1989.HereceivedtheB.S.degreeinelec-tricalengineeringfromtheUniversityofElectronicScienceandTechnologyofChina,Sichuan,China,in2012.HeiscurrentlyworkingtowardtheM.S.degreeinelectricalengineeringintheHuazhongUniversityofScienceandTechnology,Wuhan,China.

Hiscurrentresearchinterestsincludepowerelec-tronicssystemintegrationandhigh-ef ciencycon-

verters.

YongKangreceivedtheB.E.,M.E,andPh.D.de-greesfromtheHuazhongUniversityofScienceandTechnology,WuhanChina,in1988,1991,and1994,respectively.

In1994,hejoinedtheHuazhongUniversityofScienceandTechnologyasaLecturerandwaspro-motedtoanAssociateProfessor,in1996,andtoaFullProfessorin1998.HeiscurrentlytheHeadoftheCollegeofElectricalandElectronicEngineering,HuazhongUniversityofScienceandTechnology.Heistheauthorofmorethan60technicalpapers.Hisre-searchinterestsincludepowerelectronicconverter,acdrivers,electromagneticcompatibility,andtheirdigitalcontroltechniques.